The present invention relates to an improvement in exclusive coating devices used in the main for electrostatic powder coating one side of a flat object to be coated. Furthermore, the present invention relates to exclusive coating devices, that is, for coating one side only, arranged so that the bottom surface of a space in which the powder coating is performed comprises an electroconductive belt conveyer which also serves as a conveying device for objects to be coated and which is designed to be utilized in recovering oversprayed powder. Therefore, this exclusive coating device is made capable of performing at high speed, electrostatic powder coating in which the use of coating materials can be significantly more economical and a particularly uniform distribution of the coated film can be obtained. Also, the coating materials can be effectively applied to the end portions of the object to be coated, and color change can also be as completed in a significantly shorter time.
An electrostatic powder coating exhibits significantly advantageous characteristics from the viewpoints of excellent quality of the coated film, prevention of pollution, and saving of energy. Therefore, the importance of electrostatic powder coating has been further recognized recently.
In conventional electorstatic powder coating, the coating process is executed in such a manner that a material, having a surface which has not been coated and has been formed substantially to the same shape as that of the final product after the assembly work on the material has been completed, is subjected to coating by spraying a coating powder on it using an electrostatic coating powder gun while being hung from a hanger conveyer so as to be passed through a coating booth. Next, the thus-coated material is subjected to a baking process, and thus its coating process is completed.
However, such electrostatic powder coating suffers from two problems in that the coating materials are, in general, expensive, and an excessive length of time is required to change the coating color in the coating booth and the recovery device. As a result, a wide distribution of the electrostatic coating is prevented. The problem of cost of the coating material can be, in principle, satisfactory overcome by making the coated film thin. However, since in practice the materials to be coated have complicated shapes, the thickness of the coating on the projecting portion becomes too great when a predetermined coating thickness is realized due to Faradicage effect. This is a critical defect involved in powder coating. Consequently, the average coating thickness on the actual object to be coated becomes excessively greater than the thickness needed to be applied for achieving the principle object of the coating, such as shielding and rust prevention. As a solution for this problem, in the outer casing of home electric products, such as refrigerators, electric washing machines, coolers, which are manufactured by subjecting a steel plate to bending, the powder coating is arranged to be performed when the material is only in the stage of a punched out flat material before being subjected to a major bending operation. The latter working such as bending and so forth are then performed. As a result of such a method, a thin film coating exhibiting an excellent uniformity having the thickness needed in principle can be performed without any affection of the above-described Faradicage effect. As a result of this, a powder coating can be performed which is competitively satisfactory with respect to liquid coating from the viewpoint of cost of coating. In addition, the following advantages can be obtained with such a coating method: the number of times the powder coating has to be needs to be performed only once, and an excellent quality coating can be formed on the cut edge of the material. Furthermore, the quality of the appearance of the coating is suitable for home electric appliances.
The powder coating for achieving the above-described objects is classified in two ways:
A case in which only one side of the flat object needs to be subjected to the coating, while the hidden side needs to be free as much as possible from the presence of the coated film, and the cut edge of the object needs to be sufficiently coated;
another case in which a predetermined width of 5 mm to 10 mm from the cut edge of the flat object to be coated is needed to be formed on the hidden side. Furthermore, in many cases, the other regions need to be free from the coated film. In particular, the following fact is critical for performing the bending work after the coated film has been formed: that the thickness of the coated film on the cut edges and regions adjacent to the end portions is the same thickness as that of the central portion of the object to be coated. In particular, it is critical that at least the thickness of the region adjacent to the cut edges of the flat plate is not excessive with respect to that of the central portion. That is, on flat plates which have been subjected to the usual electrostatic powder coating tend to become thicker in the region in the vicinity of the cut edges thereof. Therefore, when bending work is intended to be performed along a bending line included in the surface of the flat plate, the coated film in the thickly coated portions at the two ends can generate cracks. Furthermore, the above-described powder coating critically needs to be arranged in such a manner that a change of coating color in the coating device can be completed in a short time. The usual and conventional widely distributed electrostatic powder coating equipment in which objects to be coated are hung from a hanger conveyer and the thus-hung objects are subjected to coating cannot satisfy the above-described required conditions because of the following reasons:
That is, FIG. 10 illustrates the horizontal cross-sectional shape of a flat plate object to be coated which has been manufactured by punching and subjected to an electrostatic powder coating by use of a usual powder coating device. A flat plate object 71 to be coated disposed perpendicularly to the surface of the drawing sheet is subjected to the powder coating by an electrostatic powder coating gun 77 with the same being moved in the direction designated by the arrow 80 in a coating space surrounded by booth side walls 78 and 79. The thickness of the powder layer which has been thus-formed on the object to be coated is schematically shown in this drawing. As can be clearly seen from this drawing, a powder layer 72 of uniform thickness is formed in the central portion of the object 71 to be coated. However, thick powder layers 73 and 74 are formed adjacent to the gun 77 and at both ends of the object 71 to be coated in the direction of the movement of the conveyer. The thickness of these thick powder layers 73 and 74 is greater than that of the powder layer 72 formed at the central portion of the object 71 to be coated by 25 to 40%. Furthermore, other thick layers 75 and 76 are formed on the object 71 to be coated on the side opposite from the gun 77. The width of these layers in general reaches several tens of millimeters. The electrostatic powder coating process of which the present invention is the subject is arranged in such a manner that when the required products are manufactured by forming a coated film on the flat plates 71 with a bending or assembly work being performed subsequently electrostatic powder coating process needs to have the uniformity of the coated film confined within a range of .+-.10%, or if conditions permit, .+-.20%. Therefore, the powder coating performed by the conventional electrostatic powder coating device shown in FIG. 10 cannot satisfy the above-described requirements. The reason for this lies in that the electric lines of force are concentrated in the end portions of the object to be coated 71, causing the electric field to be concentrated. As a result, a large quantity of powder is necessarily applied to this portion and the thickness of the end portions becomes excessively greater than that of the central portion.
An object of the present invention is to prevent a rise in the total cost of the powder coating material required in by electrostatic powder coating applied to a flat objective to be coated, this object being intended to be achieved by preventing an increase in the thickness of the powder coating layer at the end portions of the object to be coated with respect to that at the central portion of the same so that the quantity of the powder coating material used is limited.
Another object of the present invention is to prevent generation of cracks in a thickly coated portion at the end portions of the object to be coated when the flat plate object to be coated is bent during forming with the coated film thereon.
A further object of the present invention is to complete a color change in the coating device for performing the electrostatic powder coating in a short time.